In recent years, the notion of mounting large lithium-ion secondary batteries, having high energy density and excellent output energy characteristics, in electric vehicles has been investigated in response to increasing concern over environmental issues. In small mobile device applications such as mobile telephones or laptop computers, the capacity per unit volume is important, and therefore graphitic materials with a large density have primarily been used as active material for anodes. However, lithium-ion secondary batteries for automobiles are difficult to replace at an intermediate stage due to their large size and high cost. For this reason, lithium-ion secondary batteries for vehicles are required to have the same durability as that of vehicles, for example, a product life of 10 years or longer (high durability). Furthermore, with regard to usage conditions of secondary batteries, the charging time is, for example, 1 to 2 hours in a small portable device, whereas for a power supply for a hybrid vehicle, it is several tens of seconds considering that energy is recycled during braking. Discharging time is also several tens of seconds considering the time for which a driver steps on the accelerator, and much faster charge/discharge characteristics are required compared to a lithium-ion secondary battery for portable devices.
Conventionally, a carbonaceous material obtained by carbonizing an organic material or a vegetable raw material has been effectively used as the anode of a lithium-ion secondary battery, but excellent charge/discharge characteristics are required in the carbonaceous material for anodes used in a vehicle-mounted lithium-ion secondary battery as described above, and improvement of input/output characteristics is indispensable for realizing such a battery.
Up to now, ensuring the presence of voids in the anode active material in an anode of a non-aqueous electrolyte secondary battery has been examined in order to improve input/output characteristics. For example, as methods for ensuring the presence of voids in the anode active material, improvement of output characteristics and charge/discharge performance by using a spherical non-graphitizable carbonaceous material in the anode (Patent Document 1) and improving input/output characteristics by setting the electrode density to an appropriate value (Patent Document 2) have been described, but input/output characteristics were not sufficient. Furthermore, to ensure the presence of interparticle voids of the active material, a carbonaceous material in which particle shape and particle size distribution are adjusted has been proposed (Patent Document 3).